This invention relates in an olefin polymerization process and particularly a process for polymerization of olefins in liquid medium. The invention further relates in polymerization processes containing a prepolymerization step. This invention-also relates in an olefin polymerization apparatus.
Various methods for manufacturing solid polymers from hydrocarbons, for example from 1-olefins have been developed. In one such method olefins, such as ethylene, propylene or butene, are polymerized in the. presence of catalysts in hydrocarbon diluents or in monomers acting as diluents. The reactants are kept in liquid phase by maintaining a proper pressure in the polymerization reactor. When the polymer is insoluble or only slightly soluble in the diluent, the polymer product forms as particles suspended in the diluent and therefore the process is called a slurry process.
As batch process the above process has the advantage that all the polymer particles have the same residence time in the reactor and therefor the product quality is even. However, in commercial high production plants the polymerization reactors tend to be large. The operation is labor extensive and the quality of the products from batch to batch is not the same. For these reasons the batch reactors are not commercially acceptable.
A typical continuous slurry process is carried out in a continuous pipe reactor forming a loop, where the polymerization is carried out in a circulating turbulent flow. The product containing polymer, diluent and monomers, is taken from the loop reactor either continuously, or more usually, periodically through a discharge valve and it is introduced to a separator, where the polymer is separated by lowering the pressure.
Another reactor type in olefin polymerization art is a gas phase reactor, where polymerization is carried out in the presence of catalysts and gaseous monomers. Typically the polymerization is carried out in fluidized bed reactors, where polymerization is carried out continuouly in a bed formed by polymerizing polymer particles. This bed is kept in fluidized state by circulating gaseous flow from the top of the reactor to the bottom of the reactor. Polymerization heat is removed by cooling said circulating gaseous flow.
It is also known continuous multiphase processes, where slurry reactors, such as loop reactors are followed by one or more gas phase reactors or where two or more gas phase reactors are used in series.
A known problem in continuous processes is that even residence time of catalyst is difficult to achieve Therefore the product quality tends to be more or less uneven. This phenomena is exaggerated in multiphase processes. The catalyst is usually fed into the first reactor only. Some of the catalyst particles react with the monomers for a longer period, whereas part of the catalysts flows straight through the reactor and will be removed more or less unreacted. In the next reactor the unreacted catalyst particles react differently with monomers and resulting, among others, in uneven product quality, gels, lumps and more difficult process control.
It is also known to prepolymerize a small amount of olefin monomer with a catalyst before using these catalysts into a main polymerization reactor. Typically such prepolymerization reduces catalyst attrition and improves the resulting polymer morphology. Prepolymerized catalysts also may suspend more readily in hydrocarbon solvents, yield polymers of higher bulk density and reduce formation of lumps in gas phase reactor.
Such prepolymerization can be carried out by contacting a solid catalyst component with a small amount of olefin monomer in a suitable diluent or monomer in a vessel separate from the main polymerization reactor.
The most convenient way to prepolymerize is a continuous prepolymerization, but due to residence time distribution, part of the catalyst will not prepolymerize enough and will produce fines in the main polymerization reactor. A batch prepolymerization often reduces the catalyst activity and there is always some difference between prepolymerized catalyst batches. Some catalysts have to be prepolymerized so much that the amount of polymer may cause the catalyst handling to be too troublesome.
Some catalyst feeders feed small batches of catalyst with a cycle time of several seconds or minutes. This sometimes causes fluctuation in prepolymerization or in actual polymerization.
The common problem in all polymerization processes mentioned above is uneven residence time distribution, which leads to uneven and undesirable product quality and more difficult process control. This problem can be to some extent avoided if tubular, very long reactors having a very small diameter is used. For example in EP 0279153 a prepolymerization method is disclosed, where the prepolymerization is carried out as a plug flow. However, that kind of reactors are difficult to control and they are not suitable for high production rates because of risk of plugging and low capacity.
Therefore need exists for olefin polymerization processes where the polymerization can be carried out so that more narrow residence time distribution can be achieved and the problems arising from the uneven polymerization degree can be avoided.
The object of the present invention is to achieve a polymerization process, where the disadvantages described above can be avoided. Another object of the present invention is to achieve a process for olefin polymerization, which can be applied, as well in normal polymerization as catalyst prepolymerization. Still another object of the invention is to achieve polymerization processes where the catalyst used is prepolymerized in certain way.
Thus the invention concerns a process for polymerization of olefin monomer in fluid medium in the presence of olefin polymerizing catalyst, diluent and optional co-catalyst and donors, said process comprising the steps:
forming a fluid stream containing said catalyst,
continuously feeding said fluid stream into an elongated polymerization reactor comprising at least two successive chambers separated by dividing plates having a diameter slightly smaller than that of the polymerization reactor,
feeding into said polymerization reactor monomer and optionally co-catalyst and donor under temperature conditions to polymerize said olefin while maintaining a mixed flow in said chambers to polymerize the monomer and optional comonomer in the fluid, and
removing the resulting polymer slurry from said polymerization reactor.
The invention also concerns an apparatus for polymerization of olefin monomer and optionally other monomer in the presence of olefin polymerizing catalyst, diluent and optional cocatalyst and donors, said apparatus comprising:
means for continuously feeding a fluid stream containing said catalyst into an elongated polymerization reactor comprising at least two successive chambers separated by dividing plates having a diameter slightly smaller than that of the polymerization reactor,
means for feeding into said polymerization reactor monomer and optionally cocatalyst and donor under temperature. conditions to polymerize said olefin while maintaining a mixed flow in said chambers to polymerize the monomer and optional comonomer in the fluid, and
means for removing the resulting polymer slurry from said polymerization reactor.
The invention also concerns an apparatus for prepolymerizing olefin polymerization catalyst.
According to the invention the polymerization takes place in an elongated reactor, preferably in cylinder-like reactor, which has an inside diameter greater than that of the inlet and outlet points. The term xe2x80x9celongatedxe2x80x9d means that the length/diameter ratio of the reactor is greater than 2, preferably more than 2,5.
According to the invention the polymerization reactor comprises at least two successive chambers separated by dividing plates having a diameter slightly smaller than that of the polymerization reactor. Preferably the polymerization reactor is divided to several successive chambers.
In such polymerization reactor preferably each part of the chambers can be equipped with some mixing device in order to eliminate the deposition of the catalyst or forming polymer onto the surfaces of the polymerization reactor. As mixer device one or more rotating or static mixers can be used. The static mixers may also be attached to the walls of the reactor.
The chambers are divided from each other with plate-like members so that a narrow gap is situated between the chambers. Therefore the diameter of the plates is preferably slightly smaller than the inside diameter of the polymerization reactor.
However, it is also possible that the diameter of the dividing plates is smaller. Generally it can be said that the diameter can be 1-25 mm less than the inner diameter of the reactor. When the polymerization reactor is divided to several chambers, the diameter of the dividing plates can decrease thereby eliminating the risk of plugging. The gap between the diameter of the reactor and the diameter of the dividing plate in the last chamber is preferably smaller than the diameter of 1he outlet opening of the reactor.
According to one embodiment of the invention the dividing plates and the mixers between the dividing plates are attached to the same shaft.
Liquid and the catalyst particles flow to the next chamber between the separation plate and the wall of the reactor. Rotation of the disk keeps the area clean. In the first chamber of the reactor the catalyst mean residence time is of at least 3 times, preferably 10 times the cycle time of the catalyst feeder to assure that there is minimal fluctuation in the catalyst feed to the actual polymerization An the later chambers.
The reactor can be vertical or horizontal, although the vertical position is preferable. Plug flow behaviour in the reactor according to the invention assures that every active catalyst particle has enough prepolymer to prevent the breakage in later polymerization.
As an additional feature the reactor according to the invention can contain additional flow mixing means located in the inner surface of the reactor. Such means can, for example, be studs attached to the inner surfaces of the reactor in some or each chambers and/or the cover plate of the reactor. Such mixing elements enhance the mixing effect by forcing flow from the reactor walls towards the central parts of the reactor.
The flow mixing means can also comprise supporting bars, which support a bearing block for the central shaft. The bars are preferably locked by friction on the reactor walls or by other means.
A fluid carrier stream containing the polymerization catalyst or part of the polymerization catalyst is formed and fed into this reactor. A second stream of polymerizable monomer or monomers is also fed to the reactor. Other components of the catalyst system can also be fed to this reactor.
As fluid carrier stream inert hydrocarbon diluents can be used. Such diluents include, among others, propane, butane, pentane, hexane and alike. Also polymerizable monomers can be used as the fluid carrier. Such monomers include for example propylene, butene and hexene. The mixtures of the fluid carriers mentioned can also be used.
The polymerization takes place inside of the reactor as a mixed plug flow. This term means that the flow inside of the reactor takes place as mixed flow, the flow direction of which is, however, generally forward from one end of the reactor to other end. No part of the reactor contents flows backwards through the reactor and no stagnant flow regions exist where the flow remains in place longer than other parts of the flow.
The residence time in the reactor depends on such factors as the catalyst used or the polymerization degree desired, but generally the advantages of the invention are achieved when the residence time is more than one minute, preferably from two minutes to 30 minutes. A shorter time is sufficient when the polymerization reactor is used as a prepolymerization reactor and longer periods are necessary when the polymerization reactor is used as the ordinary polymerization reactor.
At least part of the cocatalyst is fed to the first chamber of the reactor according to the invention. The monomer or monomers can be fed to any or every chamber of the reactor. In case of propylene polymerization also the donor can be fed to any of the chambers or along with the catalyst.
The polymerization heat is removed by cooling. The cooling can be carried out with a cooling jacket surroundig the polymerization reactor. However, other methods for cooling can also be used.
The reaction temperature can be selected within a wide range, for example in the range of 0xc2x0 C. to 90xc2x0 C. The pressure can be likewise selected within a wide range, for example between 10 bar- 100 bar. The cooling jacket can be divided into several compartments to make possible to use temperature gradient over the length of the reactor.
Use of solid, transition metal-base polymerization catalysts for polymerization of olefins is well known. Typically these catalysts are based a complex derived from a halide of a transition metals, such as titanium, vanadium, chromium and zirconium, and a cocatalyst, which are typically based on metal alkyls, such as organoaluminum compounds. A typical catalyst comprises a titanium halide, which is supported on a magnesium halide complexed with an alkyl aluminum. It is also known to use electron donors or Lewis bases for controlling the stereospecifity of the polymer. Examples of such electron donors are, among the others, ethers, esters and siloxanes. Except of Ziegler-Natta catalysts described above also metallocene type catalysts can be used according to the invention.
The catalyst component to be fed to the reactor can also be mixed in appropriate medium. Such medium can be for example hydrocarbonwax. The catalyst can also be prepolymerized in conventional way and further treatment is carried out according to the invention.
The process of the invention is particularly advantageous in prepolymerization of such catalysts. Every stage of prepolymerization can be carried out in different conditions. Eg. temperature, monomer, diluent, cocatalyst and donor concentrations can be varied. Different monomers can be used in different stages of prepolymerization. Different components can be contacted in certain order to achieve optimum performance of the catalyst. Further conventional antistatic agents can be fed to the reactor in any desired point.
The design of the reactor according to the invention is easy to manufacture. No extra flanges or walls are needed which could plug the reactor. The number of chambers can changed simply by reducing or adding separation plates and mixing elements also afterwards.
The process according to the invention is particularly advantageous applied as prepolymerization step in various polymerization process. Thus one object of the invention is a process for polymerization of olefin monomer in the presence of olefin polymerizing catalyst, diluent and optional cocatalyst and donors, by feeding into at least one polymerization reactor monomer(s), diluent, catalyst, cocatalyst and optional hydrogen and/or donor under temperature conditions to polymerize said olefin(s) to olefin polymers and after polymerization removing said polymers from the reactor. The process is characterized in that the catalyst fed into the polymerization reactor is prepolymerized by forming a fluid carrier stream containing said catalyst, continuously flowing said carrier stream into an elongated prepolymerization chamber containing at least two successive chambers separated by dividing plates having a diameter slightly smaller than that of the prepolymerization chamber, feeding into said prepolymerization chamber monomer and optionally cocatalyst and donor by maintaining a mixed plug flow in said prepolymerization chamber for a period of at least one minute under temperature conditions to prepolymerize said olefin onto said catalyst, and feeding said prepolymerized catalyst into the first polymerization reactor.
Thus the prepolymerization of the catalyst is carried out first and the process further can comprise one or more slurry reactors after said prepolymerization step. The slurry reactors can be conventional stirred-tank reactors or loop reactors.
The process according to the invention can also comprise also a process where the preceding prepolymerization step is followed by one or more gas phase reactors.
Further the polymerization process following the preceding prepolymerization step can also be a combination of slurry and gas phase polymerization steps. The slurry polymerization step is preferably a loop reactor step.
In slurry processes the pressure in the prepolymerization vessel is selected preferably so that it is higher than in the following reactor. Thus the transfer of the prepolymerized catalyst from the prepolymerization chamber into the slurry reactor is as easy as possible, because the catalyst can be straight moved to the next reactor. Thus the pressure can be for instance between 40-90 bar, preferably 50-70 bar, however provided that the pressure is higher than in the next slurry reactor. The transfer of the prepolymerized catalyst can be carried out also periodically, if desired, and also conventional catalyst transfer devices can be used, if necessary.
It is recommendable that the whole catalyst amount in the process will be fed to the prepolymerization according to the invention and no extra catalysts will be fed to the slurry reactor or reactors. Instead it is possible to feed the cocatalyst either only to the prepolymerization step or partly into the prepolymerization chamber and partly to the slurry polymerization reactor or reactors.
Low boiling inert hydrocarbon is fed to the slurry reactor as polymerizing medium. Examples of suitable hydrocarbons are aliphatic hydrocarbons like propane, butane, pentane and hexane. Advantageous hydrocarbons are especially propane and isobutane. It is also possible to use a mixture of one or more hydrocarbons mentioned before. In the case of propylene polymerization the polymerization medium is preferably propylene.
The reaction mixture consisting of a reaction mixture from previous reactor together with added fresh monomer, hydrogen, optional comonomer and cocatalyst is circulated continuously through the slurry reactor, whereby more suspension of polymer in particle form in a hydrocarbon medium or monomer will be produced. The conditions of the slurry reactor will be chosen so that at least 12 w-% of the whole production will be polymerized in each slurry reactor. The temperature can be chosen within the range of 40-110xc2x0 C., advantageously within the range 50-100xc2x0 C. The reaction pressure can be chosen within the range of 40-90 bar, preferably within the range or 50-70 bar, however provided that the reaction pressure is lower than the pressure of the previous reactor. The residence time must be at least 10 minutes, but preferably in the range of 0.5-2 hours.
In slurry polymerization more than one reactors can be used in series. In such case the polymer suspension in an inert hydrocarbon or in monomer produced mi the slurry reactor is fed without the separation of inert components and monomers periodically or continuously directly to the latter slurry reactor, which acts in lower pressure than the previous slurry reactor.
Further it is possible to operate one or more slurry reactors at temperatures and pressures that are above the critical temperature and pressure of the polymerization medium. The polymerization then takes place under supercritical conditions.
The type of the slurry reactor(s) can be conventional stirred-tank reactors or loop reactors or the combination thereof. Preferably loop reactors are used.
The reactor according to the invention can be used also as a prepolymerization reactor before one or more gas phase reactors. Gas phase reactor can be an ordinary fluidized bed reactor, although other types of gas phase reactors can be used. In a fluidized bed reactor the bed consists of the formed and growing polymer particles as well as still active catalyst come along with the polymer fraction. The bed is kept in a fluidized state by introducing gaseous components, for instance monomer on such flowing rate which will make the particles act as a fluid. The fluidizing gas can contain also inert carrier gases, like nitrogen and also hydrogen as a modifier.
The gas phase reactor used can operate at temperature region between 50-115xc2x0 C., preferably between 60-110xc2x0 C. and the reaction pressure between 10-40 bar and the partial pressure of monomer between 2-30 bar.
According to still another embodiment the reactor of the invention is used as a prepolymerization reactor which is followed by a slurry reactor or reactors and a gas phase reactor or reactors.
In every polymerization step it is possible to use also comonomers selected from ethylene, propylene, butene, pentene, hexene and alike as well as their mixtures.